[GitHub] [flink-ml] zhipeng93 commented on a diff in pull request #216: [FLINK-31173] Fix serveral iteration bugs

["zhipeng93 (via GitHub)" <gi...@apache.org>]

zhipeng93 commented on code in PR #216:
URL: https://github.com/apache/flink-ml/pull/216#discussion_r1172105766


##########
flink-ml-tests/src/test/java/org/apache/flink/test/iteration/BoundedTailOperatorWithUnionInputIterationITCase.java:
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@@ -0,0 +1,75 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.flink.test.iteration;
+
+import org.apache.flink.iteration.DataStreamList;
+import org.apache.flink.iteration.IterationBody;
+import org.apache.flink.iteration.IterationBodyResult;
+import org.apache.flink.iteration.IterationConfig;
+import org.apache.flink.iteration.IterationConfig.OperatorLifeCycle;
+import org.apache.flink.iteration.Iterations;
+import org.apache.flink.iteration.ReplayableDataStreamList;
+import org.apache.flink.ml.common.iteration.TerminateOnMaxIter;
+import org.apache.flink.streaming.api.datastream.DataStream;
+import org.apache.flink.streaming.api.environment.StreamExecutionEnvironment;
+
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+/** Tests the case that Tail operator contains multiple inputs. */
+public class BoundedTailOperatorWithUnionInputIterationITCase {
+
+    @Test
+    public void testTailOperatorWithUnionInput() {
+        StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();
+        DataStream<Long> input1 = env.fromSequence(0, 10);
+        DataStream<Long> input2 = env.fromSequence(0, 10);
+        String expectedErrorMessage =
+                "Tail operator should have only one input. Please check whether operator "
+                        + "\"Union\" contains multiple inputs.";
+
+        try {
+            Iterations.iterateBoundedStreamsUntilTermination(
+                    DataStreamList.of(input1),
+                    ReplayableDataStreamList.replay(input2),
+                    new IterationConfig(OperatorLifeCycle.PER_ROUND),
+                    new IterationBodyWithUnionAsFeedback());
+        } catch (UnsupportedOperationException e) {
+            assertEquals(expectedErrorMessage, e.getMessage());
+        }
+    }
+
+    private static class IterationBodyWithUnionAsFeedback implements IterationBody {
+
+        @Override
+        public IterationBodyResult process(
+                DataStreamList variableStreams, DataStreamList dataStreams) {
+
+            DataStream<Long> variableStream = variableStreams.get(0);
+            DataStream<Long> constantStream = dataStreams.get(0);
+            DataStream<Integer> terminationCriteria =
+                    constantStream.flatMap(new TerminateOnMaxIter<>(3));
+            return new IterationBodyResult(
+                    DataStreamList.of(variableStream.union(constantStream)),

Review Comment:
   I am afraid adding `map(x -> x)` to the wrapped operator does not work here. Currently iterations do not support `watermark` alignment. The bug comes in because `TailOperator` does not align the `epochwatermark`, rather than watermarks. `epochwatermark` should be viewed as a normal record from Flink framework's view.
   
   The current situation is that:
   - The tailOperator sends the epochwatermark to headOperator.
   - headOperator expects that the epochwatermark should be strictly increasing.
   - The tail operator made an assumption that it has only one input and it does not align the epochwatermarks. When receiving epochwatermarks, it directly pushes to the head operator.
   
   If the tail operator has two inputs, it will push two same epochwatermarks to headOperator, which breaks the assumption.
   
   I also added a TODO there.



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